How do officials investigate vehicle fires?

A white stretch limousine carrying nine women to a bachelorette party burst into flames Saturday night, killing the bride-to-be and four other passengers. Despite witness accounts of the accident on the San Mateo Bridge in the San Francisco Bay area, officials are still unsure how the deadly limousine fire started. How do investigators determine what caused a fire in a vehicle?

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Through examination and elimination. Investigators start by examining the vehicle as it stands after the fire. Three main zones are examined: the engine compartment, the passenger compartment, and the trunk. Each area is divided into sections; damage is compared from section to section in each of the zones.

In the engine compartment, investigators look for localized damage and spills from different fluids (such as brake fluid or transmission fluid) in each section to determine where the fire originated. In the passenger compartment, five sections are examined closely for clues: the front, back, left, right, and console. (Nowadays, drivers carry so many items in the console that it receives its own separate investigation.) In extended vehicles, like limousines, several additional sections exist within each zone.

Burn marks are key to this process. For example, V-shaped patterns are associated with initial flames - a telltale sign of where the fire began. By examining what was burned and what wasn't, in addition to specific burn patterns, investigators can gather more information about what happened. After the vehicle is towed from the scene of the fire, the patterns on the ground can provide additional clues.

If an investigator observes a deeper burn on one of the seats in the vehicle, part of the seat will be cut out and taken in for testing for ignitable fluids. Investigators also note if there are any strange odors that could indicate the presence of ignitable fluid.

The vehicle's history also can provide clues to the cause of the fire. If the car's model was recalled in the past, investigators will take note. Details such as the presence of customer-installed features are also important - modifications may have been installed incorrectly and become potential sources of ignition.

Investigators may also use supplemental evidence, such as accounts from surviving witnesses, if there are any. Details like an odd smell, smoke or even prior malfunctions with the electrical system are highly telling. Information from passersby, as well as any photos or videos from surveillance cameras, are helpful in reconstructing a timeline of the incident.

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Explainer thanks Capt. Anthony Czepik of the Fire Investigation Bureau of the City of Baltimore and Dr. John D. De Haan, co-author of Kirk's Fire Investigation.